Conversion of low molecular weight alkanes such as methane to synthetic fuels or chemicals has received increasing attention because low molecular weight alkanes are generally available from secure and reliable sources. For example, natural gas wells and oil wells currently produce vast quantities of methane. Reported methods for converting low molecular weight alkanes to more easily transportable liquid fuels and chemical feedstocks can be conveniently categorized as direct oxidative routes and/or as indirect syngas routes. Direct oxidative routes convert lower alkanes to products such as methanol, gasoline, and relatively higher molecular weight alkanes. In contrast, indirect syngas routes typically involve production of synthesis gas as an intermediate product.
Routes are known for converting methane to dimethyl ether. For example, methane is steam reformed to produce synthesis gas. Thereafter, dimethyl ether and methanol can be manufactured simultaneously from the synthesis gas, as described in U.S. Pat. No. 4,341,069 issued to Bell et al. They recommend a dimethyl ether synthesis catalyst having copper, zinc, and chromium co-precipitated on a gamma-alumina base. Alternatively, methane is converted to methanol, and dimethyl ether is subsequently manufactured from methanol by passing a mixed vapor containing methanol and water over an alumina catalyst, as described in an article by Hutchings in New Scientist (Jul. 3, 1986) 35.
Formaldehyde is a very important intermediate compound in the chemical industry. The extreme reactivity of the formaldehyde carbonyl group and the nature of the molecule as a basic building block has made formaldehyde an important feedstock for a wide variety of industrially important chemical compounds. Historically, formaldehyde has found its largest volume of application in the manufacture of phenol-formaldehyde resins, urea-formaldehyde resins and other polymers. Pure formaldehyde is quite uncommon since it polymerizes readily. It was usually obtained as an aqueous solution such as formalin, which contains only about 40 percent formaldehyde. However, more recently, formaldehyde is usually transported as an item of commerce in concentrations of 37 to 50 percent by weight. A solid source of formaldehyde called paraformaldehyde is also commercially available.
Because of the reactivity of formaldehyde, its handling and separation require special attention. It is a gas above -19.degree. C. and is flammable or explosive in air at concentrations of about 7 to about 12 mol percent. Formaldehyde polymerizes with itself at temperatures below 100.degree. C. and more rapidly when water vapor or impurities are present. Since formaldehyde is usually transported in aqueous solutions of 50 percent by weight or lower concentration, producers have tended to locate close to markets and to ship the methanol raw material, which has a smaller volume.
It is known that some reactions may be carried out by means of catalytic distillation. In catalytic distillation, reaction and separation are carried out simultaneously in a distillation column with internal and/or external stages of contact with catalyst.
In U.S. Pat. No. 4,215,011, Smith, Jr. discloses a method for the separation of an isoolefin, preferably having four to six carbon atoms, from streams containing mixtures thereof with the corresponding normal olefin, wherein the mixture is fed into a reaction-distillation column containing a fixed-bed, acidic cation exchange resin and contacted with the acidic cation exchange resin to react the isoolefin with itself to form a dimer and the dimer is separated from the normal olefin, the particulate catalytic material, i.e., the acidic cation exchange resin, being contained in a plurality of closed cloth pockets, which pockets are arranged and supported in the column by wire mesh.
In U.S. Pat. No. 4,443,559, Smith, Jr. discloses a catalytic distillation structure which comprises a catalyst component associated intimately with or surrounded by a resilient component, which component is comprised of at least 70 vol. percent open space for providing a matrix of substantially open space. Examples of such resilient component are open-mesh, knitted, stainless wire (demister wire or an expanded aluminum); open-mesh, knitted, polymeric filaments of nylon, Teflon, etc.; and highly-open structure foamed material (reticulated polyurethane).
In U.S. Pat. No. 5,113,015, David A. Palmer, K. D. Hansen and K. A. Fjare disclose to a process for recovering acetic acid from methyl acetate wherein the methyl acetate is hydrolyzed to methanol and acetic acid via catalytic distillation.
In German Democratic Republic DD 245 868 A1 published May 20, 1987 in the text submitted by the applicant, preparation of methylal is carried out by reaction of methanol with trioxane, formalin or paraformaldehyde in the presence of a specific zeolite. Autoclave reactions of 1 to 8 hours are described using a zeolite of the "LZ40 type" with a ratio of silicon dioxide to alumina ratio of 78 at temperatures from 493 to 543 K. Methylal content of the product as high as 99.8 percent (without methanol) is reported for trioxane at 523 K for 3 hours. Reaction pressures did not exceed 5 Mpa in the autoclave. Neither conversions nor selectivity are reported.
In U.S. Pat. No. 4,967,014, Junzo Masamoto, Junzo Ohtake and Mamoru Kawamura describe a process for formaldehyde production by reacting methanol with formaldehyde to form methylal, CH.sub.3 OCH.sub.2 OCH.sub.3, and then oxidizing the resulting methylal to obtain formaldehyde. In the methylal formation step, a solution containing methanol, formaldehyde and water was brought into solid-liquid contact with a solid acid catalyst, and a methylal-rich component was recovered as a distillate. This process employs reactive distillation performed using a distillation column and multireaction units. The middle portion of the distillation column was furnished with stages from which the liquid components were withdrawn and pumped to the reactor units, which contained solid acid catalyst. The reactive solutions containing the resulting methylal were fed to the distillation column, where methylal was distilled as the overhead product.
Polyoxymethylene dimethyl ethers are the best known members of the dialkyl ether polymers of formaldehyde. While diethyl and dipropyl polyoxymethylene ethers have been prepared, major attention has been given to the dimethyl ether polymers. Polyoxymethylene dimethyl ethers make up a homologous series of polyoxymethylene glycol derivatives having the structure represented by use of the type formula indicated below: EQU CH.sub.3 O(CH.sub.2 O).sub.n CH.sub.3
Chemically, they are acetals closely related to methylal, CH.sub.3 OCH.sub.2 OCH.sub.3, which may be regarded as the parent member of the group in which n of the type formula equals 1. They are synthesized by the action of methanol on aqueous formaldehyde or polyoxymethylene glycols in the presence of an acidic catalyst just as methylal is produced. On hydrolysis they are converted to formaldehyde and methanol. Like other acetals, they possess a high degree of chemical stability. They are not readily hydrolyzed under neutral or alkaline conditions, but are attacked by even relatively dilute acids. They are more stable than the polyoxymethylene diacetates.
Due to the relatively small differences in the physical properties (melting points, boiling points, and solubility) of adjacent members in this series, individual homologs are not readily separated. However, fractions having various average molecular weight values have been isolated. The normal boiling point temperature of a fraction having average n of 2 in the type formula is reported as 91.degree. to 93.degree. C. Boiling points at atmospheric pressure calculated from partial pressure equations range from 105.0.degree. C. for n of 2, to 242.3.degree. C. for n of 5. (Walker, Joseph Frederic, "Formaldehyde", Robert E. Krieger Publishing Co., issued as No. 159 of American Chemical Society Monograph series (1975), pages 167-169)
Polyoxymethylene dimethyl ethers are prepared in laboratory scale by heating polyoxymethylene glycols or paraformaldehyde with methanol in the presence of a trace of sulfuric or hydrochloric acid in a sealed tube for 15 hours at 150.degree. C., or for a shorter time (12 hours) at 165.degree. to 180.degree. C. Considerable pressure is caused by decomposition reactions, which produce carbon oxides, and by formation of some dimethyl ether. The average molecular weight of the ether products increases with the ratio of paraformaldehyde or polyoxymethylene to methanol in the charge. A high polymer is obtained with a 6 to 1 ratio of formaldehyde (as polymer) to methanol. In these polymers, the n value of the type formula CH.sub.3 O(CH.sub.2 O).sub.n CH.sub.3 is greater than 100, generally in the range of 300 to 500. The products are purified by washing with sodium sulfite solution, which does not dissolve the true dimethyl ethers, and may then be fractionated by fractional crystallization from various solvents.
U.S. Pat. No. 2,449,469 in the names of W. F. Gresham and R. E. Brooks reported obtaining good yields of polyoxymethylene dimetbyl ethers containing 2 to 4 formaldehyde units per molecule. This procedure is carried out by heating methylal with paraformaldehyde or concentrated formaldehyde solutions in the presence of sulfuric acid.
In the past, various molecular sieve compositions, natural and synthetic, have been found to be useful for a number of hydrocarbon conversion reactions. Among these are alkylation, aromatization, dehydrogenation and isomerization. Among the sieves which have been used are Type A, X Y and those of the MFI crystal structure as shown in "Atlas of Zeolite Structure Types," Second Revised Edition, 1987, published on behalf of the Structure Commission of the International Zeolite Associates and incorporated by reference herein. Representative of the last group are ZSM-5 and AMS borosilicate molecular sieves.
Prior art developments have resulted in the formation of many synthetic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Exemplary of these materials are Zeolite A (Milton, in U.S. Pat. No. 2,882,243), Zeolite X (Milton, in U.S. Pat. No. 2,882,244), Zeolite Y (Breck, in U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (Argauer, et al., in U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (Chu, in U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (Rosinski, et al., in U.S. Pat. No. 3,832,449), and others.
It is well known that internal combustion engines have revolutionized transportation following their invention during the last decades of the 19th century. While others, including Benz and Gottleib Wilhelm Daimler, invented and developed engines using electric ignition of fuel such as gasoline, Rudolf C. K. Diesel invented and built the engine named for him which employs compression for autoignition of the fuel in order to utilize low-cost organic fuels. Development of improved diesel engines for use in automobiles has proceeded hand-in-hand with improvements in diesel fuel compositions, which today are typically derived from petroleum. Modern high performance diesel engines demand ever more advanced specification of fuel compositions, but cost remains an important consideration.
Even in newer, high performance diesel engines combustion of conventional fuel produces smoke in the exhaust. Oxygenated compounds and compounds containing few or no carbon-to-carbon chemical bonds, such as methanol and dimethyl ether, are known to reduce smoke and engine exhaust emissions. However, most such compounds have high vapor pressure and/or are nearly insoluble in diesel fuel, and they have poor ignition quality, as indicated by their cetane numbers. Furthermore, other methods of improving diesel fuels by chemical hydrogenation to reduce their sulfur and aromatics contents, also causes a reduction in fuel lubricity. Diesel fuels of low lubricity may cause excessive wear of fuel injectors and other moving parts which come in contact with the fuel under high pressures.
Recently, U.S. Pat. No. 5,746,785 in the names of David S. Moulton and David W. Naegeli reported blending a mixture of alkoxy-terminated poly-oxymethylenes, having a varied mixture of molecular weights, with diesel fuel to form an improved fuel for autoignition engines. Two mixtures were produced by reacting paraformaldehyde with (i) methanol or (ii) methylal in a closed system for up to 7 hours and at a temperatures of 150.degree. to 240.degree. C. and pressures of 300 psi to 1,000 psi to form a product containing methoxy-terminated poly-oxymethylenes having a molecular weight of from about 80 to about 350 (polyoxymethylene dimethyl ethers). More specifically, a 1.6 liter cylindrical reactor was loaded with a mixture of methanol and paraformaldehyde, in molar ratio of about 1 mole methanol to 3 moles paraformaldehyde, and in a second preparation, methylal (dimethoxymethane) and paraformaldehyde were combined in a molar ratio of about 1 mole methylal to about 5 moles paraformaldehyde. In the second procedure, a small amount of formic acid, about 0.1 percent by weight of the total reactants, was added as a catalyst. The same temperatures, pressures and reaction times are maintained as in the first. Disadvantages of these products include the presence of formic acid and thermal instability of methoxy-terminated poly-oxymethylenes under ambient pressure and acidic conditions.
There is, therefore, a present need for catalytic processes to prepare oxygenated organic compounds, particularly polyoxymethylene dimethyl ethers, which do not have the above disadvantages. An improved process should be carried out advantageously in the liquid phase using a suitable condensation-promoting catalyst system, preferably a molecular sieve based catalyst which provides improved conversion and yield. Such an improved process which converts lower value compounds to higher polyoxymethylene dimethyl ethers would be particularly advantageous. Dimethyl ether is, for example, less expensive to produce than methanol on a methanol equivalent basis, and its condensation to polyoxymethylene dimethyl ethers does not produce water as a co-product.
The base diesel fuel, when blended with such mixtures in a volume ratio of from about 2 to about 5 parts diesel fuel to 1 part of the total mixture, is said to provide a higher quality fuel having significantly improved lubricity and reduced smoke formation without degradation of the cetane number or smoke formation characteristics as compared to the base diesel fuel.
This invention is directed to overcoming the problems set forth above in order to provide Diesel fuel having improved qualities. It is desirable to have a method of producing a high quality diesel fuel that has better fuel lubricity than conventional low-sulfur, low-aromatics diesel fuels, yet has comparable ignition quality and smoke generation characteristics. It is also desirable to have a method of producing such fuel which contains an additional blended component that is soluble in diesel fuel and has no carbon-to-carbon bonds. Furthermore, it is desirable to have such a fuel wherein the concentration of gums and other undesirable products is reduced.